If you have a flat file, electric drill and a dial or digital caliper or a micrometer, it's an easy chore to modify a .32 caliber jag.
Lightly tighten the .32 caliber jag's threads in the drill's chuck. With the electric drill pointed from your left to your right, start it and use the flat file to remove the excess material from the jag. For a .30 caliber bore, you want to reduce the size so it is .020 to .030 smaller than the bore.
For a .30 caliber bore, that would make the jag .270-.280 in diameter.